The present invention relates generally to an eddy currect reduction brake apparatus for large sized vehicles or the like, and, more particularly, to an eddy current reduction brake apparatus in which magnetic flux of a permanent magnet is effectively applied to a brake drum.
In conventional eddy current reduction braking systems, ferromagnetic plates have substantially rectangular cross sections and outer and inner surfaces of substantially the same area. Consequently, magnetic flux from the magnets is not concentrated by the ferromagnetic plates onto a brake drum. If the ferromagnetic plates are shaped as trapezoids, and the area of outer surfaces made narrower than the areas of inner surfaces, it is possible to concentrate the magnetic flux entering the brake drum and thereby increase magnetic flux density and enhance brake force. However, in the aforementioned construction, the edge effect concentration of magnetic flux is difficult to obtain, and it is difficult to cast the ferromagnetic plates into an outer tubular portion of a guide tube made of aluminum. Furthermore, even if the guide tube is manufactured by forging, allowance for machining of the outer and inner peripheral surfaces of the outer tubular portion increases and the volume of the ferromagnetic plates decreases because of a resultant parting line. In eddy current reduction braking systems having an immovable magnet support tube and a movable magnet support tube axially arranged in a hollow portion of a guide tube, a non-braking condition produces on the brake drum a dragging torque. Therefore, it is necessary to thicken the ferromagnetic plates and increase the volume in order to suppress the dragging torque, which is dis-advantageous in terms of braking performance.
In eddy current reduction braking systems in which a good conductor such as copper is coated or deposited on the opposite ends of a brake drum, brake force can be enhanced by ferromagnetic plates of rectangular shape and without roundness at corner portions. However, the entire periphery of the sides of the ferromagnetic plates have to be machined and a parting line eliminated by machining. Thus, even if the ferromagnetic plates are cast into a guide tube made of aluminum, the ferromagnetic plates are attracted by the brake drum such that the ferromagnetic plates can possibly move out of the guide tube. Furthermore, the guide tube contracts substantially during a cooling period after ferromagnetic plates have been cast, and cracks possibly occur in portions adjacent to corner portions of the ferromagnetic plates in the guide tube.
In the conventional eddy current reduction braking systems disclosed in Japanese Patent Laid-Open Nos. 6-38504 and 6-38505 publications or the like, side sections of ferromagnetic plates are shaped such that an outer surface of a rear portion protrudes rearwardly in a rotational direction of a brake drum, and a rear surface is inclined in a direction opposite to the rotational direction of the brake drum. The above-described construction is intended to disperse magnetic flux from the magnets to the brake drum to enhance the braking performance. However, it has been found that at the high speed rotation of the brake drum, the magnetic flux exerted on the brake drum is concentrated, rather than being dispersed, so as to enhance the edge effect of the ferromagnetic plates, and contribute to an increase in braking force.